The present invention relates to a method for performing shifts in a gearbox, in particular a planetary-type automatic gearbox, which has multiple non-positively engaging shift elements and at least one positively engaging shift element.
In conventional automatic gearboxes, the individual gear stages are shifted by actuation of frictionally engaging shift elements (for example, multiplate clutches or multiplate brakes). Those shift elements which are not actuated, that is to say are open, in the individual gears generate certain drag torques, which has an adverse effect on gearbox efficiency. It has therefore already been considered to replace individual frictionally engaging shift elements of automatic gearboxes with positively engaging shift elements, where this is technically actually possible.
It is an object of the invention to provide a method for performing shifts in a gearbox under load, which gearbox has multiple non-positively engaging shift elements and at least one positively engaging shift element.
This and other objects are achieved in accordance with embodiments of the invention.
The starting point of the invention is a gearbox, in particular a planetary-type automatic gearbox, which has multiple non-positively engaging shift elements and at least one positively engaging shift element.
The non-positively engaging shift elements may be, for example, frictional shift elements, in particular friction clutches or friction brakes such as, for example, multiplate clutches or multiplate brakes. The at least one positively engaging shift element may be, for example, a shift claw.
An analysis of different planetary-type automatic gearbox topologies has shown that, in the case of some gearboxes, one and the same transmission ratio (hereinafter referred to as “identical transmission ratio”) between the gearbox input and the gearbox output can be realized by way of different “shift states” of the individual shift elements of a gearbox. In this case, a “gearbox shift state” is defined by the present shift states of the individual shift elements of the gearbox. Each of the individual gearbox shift states has a very particular transmission ratio between a gearbox input and a gearbox output.
In a gearbox according to the invention, there are now at least two gearbox shift states which have an “identical transmission ratio”. It is, for example, conceivable for the so-called “direct gear”, which is distinguished by the fact that the rotational speed of the gearbox output is equal to the rotational speed of the gearbox input, to be realized by way of two or more different gearbox shift states.
The expression “identical transmission ratio” should be interpreted not in an absolutely literal sense but as encompassing not only mathematically exactly identical transmission ratios but also transmission ratios which are “approximately” equal, in particular transmission ratios which differ from one another by less than 10%, preferably by less than 5% and particularly preferably by less than 2%.
It may now be provided that, to shift into a first of the at least two gearbox shift states with an “identical transmission ratio”, only non-positively engaging shift elements have to be closed. In this context, however, the plural term “non-positively engaging shift elements” should not be interpreted in the strictly grammatical sense. What this means is that, to shift into a first of the at least two gearbox shift states with “identical transmission ratio”, either only one single non-positively engaging shift element or only multiple non-positively engaging shift elements (but no positively engaging shift elements) have to be closed.
However, to shift into a second of the at least two gearbox shift states with identical transmission ratio, at least one positively engaging shift element must be closed.
An aspect of the invention is that, at least in the case of some shift processes in which it is intended to shift from a present gearbox shift state, whose transmission ratio differs from the abovementioned “identical transmission ratio”, to the “identical transmission ratio”:
a) a shift is performed firstly into the first gearbox shift state and
b) subsequently into the second gearbox shift state, wherein it is pointed out that the transition into the abovementioned shift state b) does not imperatively assume a rotational speed differential of zero at the claw. The claw may be engaged even in the presence of a slight, but not excessive, rotational speed differential (engagement rotational speed differential range in which shifting can be performed).
A very great advantage of the invention can be seen in the fact that, despite the use of a positively engaging shift element, shifts can be performed under load (so-called “powershifts”).
At least in the case of some shift processes in which a direct shift into the second of the at least two gearbox shift states would not be possible under load (owing to the positively engaging shift element that has to be closed), a shift is thus initially performed into the first of the at least two gearbox shift states.
Subsequently, the shift is performed from the first gearbox shift state, which has the same transmission ratio as the second of the at least two gearbox shift states, into the second gearbox shift state.
If a shift were to be performed directly from a present gearbox shift state, the transmission ratio of which differs from the “identical transmission ratio”, into the second gearbox shift state, grating of the positively engaging shift element that has to be closed, and/or a drop in traction power, could occur at least in the case of some shift processes of the gearbox (owing to the fact that a power shift cannot be realized (open shift)).
For this reason, it may be expedient to provide in the shift strategy an “indirect route” via the first of the at least two gearbox shift states with identical transmission ratio.
Even though the at least two gearbox shift states have the same transmission ratio, it is expedient, after the shift into the first of the at least two gearbox shift states, for an onward shift to be performed into the second of the at least two gearbox shift states, because after the engagement of the respective positively engaging shift element, no further power or energy, or only a small amount of further power or energy, is required to keep said shift element closed. A transition to the second of the at least gearbox shift states with identical transmission ratio is expedient, in particular, when the gearbox is subsequently shifted onward into gearbox shift states in which the at least one positively engaging shift element has to be closed, or kept closed, in any case.
In one refinement of the invention, the above-described shift strategy is in the case of a “traction upshift”, in the case of which, during traction operation of the vehicle, it is intended to shift from a present gearbox shift state, whose transmission ratio differs from the identical transmission ratio, to the identical transmission ratio, a shift is performed firstly into the first gearbox shift state and, subsequently, into the second gearbox shift state. A traction upshift is to be understood to mean a shift process in which the drive engine and thus also the gearbox of the vehicle are in traction operation (that is to say are driving the vehicle) and a shift is to be performed from a present forward gear into a higher forward gear. A traction upshift is encountered for example if, while the vehicle is accelerating, a shift is performed from the second gear to the third gear or from the third gear to the fourth gear or from the fourth gear to the fifth gear, etc.
If two or more different gearbox shift states exist for the target transmission ratio or the target gear ratio, specifically a gearbox shift state which can be attained proceeding from the present gearbox shift state by closing only non-positively engaging shift elements, and another gearbox shift state which can be attained by closing at least one positively engaging shift element, it is expedient to implement the above-described method according to the invention because, with the method, shifting without an interruption in traction power is possible even in the case of a traction upshift in which at least one positively engaging shift element ultimately has to be closed.
In a refinement of the invention, it may be provided that, in the case of the traction upshift described above, it is always the case that a shift is performed firstly into the first gearbox shift state and, subsequently, into the second gearbox shift state. However, it is not imperative that the method according to the invention is always used in the case of every traction upshift such as is described above.
Even though traction downshifts into the claw do not have to be performed via the “alternative gear”, but can also be performed without an interruption in traction power even with any conventional claw, the method according to the invention may also be implemented in the case of traction downshifts. Specifically, it may be provided that, in the case of a traction downshift, in the case of which, during traction operation of the vehicle, it is intended to shift from a present gearbox shift state, whose transmission ratio differs from the “identical transmission ratio”, to the identical transmission ratio, a shift is performed firstly into the first gearbox shift state (that is to say by closing only one or more non-positively engaging shift elements) and subsequently into the second gearbox shift state (by closing at least one positively engaging shift element).
It may be provided that, in the case of every traction downshift as described above, it is always the case that a shift is performed firstly into the first gearbox shift state and, subsequently, into the second gearbox shift state. This is, however, not imperatively necessary.
In the case of shifts performed in overrun operation, that is to say in the case of overrun upshifts or overrun downshifts, in the case of which, during overrun operation of the vehicle, it is intended to shift from a present gearbox shift state, the transmission ratio of which differs from the “identical” transmission ratio, to the identical transmission ratio, provision may be made for a shift to be performed directly into the second gearbox shift state.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.